Log-periodic antenna structure



3 Sheets-Sheet 1 mm vm mm hmlm mm INVE/VTUR.

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J. W. GREISER LOG-PERIODIC ANTENNASTRUCTURE Feb. 13, 1968 Filed Jan. 18, 1965 i I l 1968 J. w. GREISER 3,369,243

LOG'PERIODIC ANTENNA STRUCTURE Filed Jan. 18, 1965 5 Sheets-Sheet 2 I NVEN7 OR. I

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ATTORNEYS.

Feb. 13, 1968 J. w. GREISER 3,

LOG-PERIODIC ANTENNA STRUCTURE Filed Jan. 18, 1965 3 Sheets-Sheet 5 INVENTOR.

ATTORNEYS.

United States Patent O 3,369,243 LOG-PERIODIC ANTENNA STRUCTURE John W. Greiser, Champaign, Ill., assignor to University of Illinois Foundation, Urbana, Ill. Filed Jan. 18, 1965, Ser. No. 426,236 11 Claims. (Cl. 343-770) ABSTRACT OF THE DISCLOSURE A wide band folded element type antenna structure having a plurality of series fed folded elements varying in length and spacing, and a series of phasing elements allowing control of the phase progression of the element currents. Three distinct modes of operation-a folded slot, dipole or monopole array-can be established.

This invention relates to antenna arrays of the wideband back-fire variety.

In the past, efforts have been made to provide antenna structures which are substantially frequency independent over a wide range of frequency. Substantial success already has been had. However, the antennas heretofore used have had a limited amount of symmetry and the present proposal is based on improving symmetry, radiation patterns and impedance characteristics.

To achieve the results of the present invention, recourse has been had generally to a folded-element type of structure. The structure may be either a dipole, monopole or slot-type. As will be appreciated from What is to follow, the slot-type structure is essentially a photographic negative of the folded dipole, and the monopole is essentially one-half of the folded dipole type.

All antennas of the character herein to be discussed are usable over an extremely wide band of frequency to an extent that they are substantially frequency independent. The range of frequencies depends generally upon the maximum and minimum lengths of the elements and the rate at which different elements vary with respect to each other. Generally speaking, all elements are coupled to some form of transmission line. All elements extend outwardly from such transmission line and terminate on an imaginary line which can be assumed to represent some selected angle extending outwardly from the center line of the transmission line. This angle usually is in the range between about as a minimum and about 50 as a maximum.

The folded elements are arranged so as to extend outwardly until some selected point of the folded element falls approximately on the selected angular path. The folded elements are scaled relative to each other by a selected scale factor which in the usual case is related to the chosen angle of the outer edge or limit of the elements relative to each other and to the transmission line.

The broad purpose of such a scaling factor to establish the relative length relationship between antenna sections .or elements is known in the art. It has already been explained in an application for Letters Patent filed on May 3, 1960 by Dwight E, Isbell, Ser. No. 26,589, now Patent No. 3,210,767. It was also discussed in the United States PatentNo. 3,108,280 issued to Paul E. Mayes and Robert L. Carrel on Oct. 22, 1963. This constant scale factor has a value which is less than unity. It is preferably in the range of about 0.65 to about 0.98, although the stated limits may vary slightly where desired. Essentially, the scale factor is represented by the ratio of the corresponding lengths of two adjacent elements to each other, the smaller of the two lengths being the numerator and the larger of the two the denominator of the ratio factor.

Similarly, the scale factor can be used to represent and designate the spacing between any two adjacent elements of such an antenna array. In a true log-periodic antenna combination, the scale factor which establishes the ratio of the lentgh of the several elements, that is the distance the elements extend outwardly from the feeder, is identical to that same scale facor which determines the spacing between adjacent elements. As was the case of the scale factor defining the element length, the scale factor for defining element spacing is the ratio of the space or the separating distance between a given element and the next smaller element to the spacing between the same element and the next larger element of the antenna combination. Under such conditions and to establish the ratio, it is apparent that the antenna combination should consist of at least three elements to provide a true log-periodic assembly.

While under optimum operation it is usually desirable that the scale factor determining the element lengths shall be the same scale factor that determines element spacing, it happens, at times, that for practical reasons the scale factors may be somewhat different. Usually, the antenna efficiency is reduced to a minor extent for such differences as compared to the efficiency which would be had to true log-periodic operations.

In regard to the possibility that the scale factors for determining the element length and element spacing are different, it usually, although not necessarily, results in a case where the scale factor determining element spacing is a closer approach to unity. This is often done for manufacturing convenience or for special radiation characteristics. It happens that for very limited bands of frequency an element spacing factor near unity can result' in improved performance. Band width, in effect, is traded off for narrower radiation patterns.

The antenna here described comprises not only the folded elements whose lengths and spacings are established in accordance with the scale factors, but, in addition, comprises a series of phasing elements located within the folded elements thereby allowing individual control of the phase progression of the element currents. The .precise control of the antenna phasing results in a substantial improvement in performance.

It will be apparent from what is stated that the antenna structure herein to be described is what may be termed a log-periodic folded-slot, dipole, or monopole array. In the slot form, this combination is applicable to structures which are capable of being flush-mounted. In this respect the antenna is particularly desirable for use in connection with aircraft, missiles, or hardened site situations, or it can be formed or supported directly on the surface of the carrying component. It also can be formed from the general technique of printed circuitry so that it may be fabricated at a minimum of expense and thereby provide particularly satisfactory and stable operation, up to and exceeding frequencies of 10 c.p.s. (10 kmc.).

The antenna structure described herein is of a type that is distinguishable from any other log-periodic design in that three distinct modes of operation may be established. In one, the operation will be a slot array which can be mounted flush with the ground and capable of bilaterally producing vertically polarized unidirectional radiation. It also can provide a form of linearly polarized free-space antenna. Still further, the structure to be described can provide a vertically polarized monopole array over ground. The radiation patterns of the three forms are the same after taking into account the proper symmetry relationships between forms. The wide versatility comes into being because of these symmetry properties of the structure as set forth.

The antenna is certain of its preferred forms is depicted by the accompanying drawings wherein FIG. 1 shows, for instance, a log-periodic folded-slot array comprising, as illustrated, four element sections; FIG. 2 is a I representation of a 1og-periodic folded dipole array; FIG.

3 is a showing of a log-periodic folded monopole ar-ray; FIG. 4 is a schematic illustration of a typical arrangement for supporting a log-periodic folded monopole array of the general type shown by FIG. 3; FIG. 5 is a structure which is a modification of that shown in FIG. 4; FIG. 6 is a schematic showing of a structural modification providing a unilateral far-field cancellation obtained by the combination of slot and a loop structure; FIG. 8 is a schematic representation of an absorbing cavity placed behind a log-periodic folded-slot array of the general type shown illustratively by FIG. 1; and FIG. 7 is a schematic showing of a modification of the folded monopole array wherein a triangularly shaped tooth version of the antenna structure is utilized.

Referring now to the drawings, first, FIG. 1 depicts a schematic illustration of a typical log-periodic foldedslot array. This is a form of antenna particularly adapted for flush mounting, as in the already suggested missile and aircraft fields. :In this form of arrangement, the antenna elements are formed on a conducting surface 11. The various slots are provided, as shown at 13, by removing certain of the conducting material to form an outline which is substantially the negative of the folded dipole array. The slots themselves are formed from complementary slot sections on either side of central connecting passages, such as and 15', separating adjacent slot elements, such as 13 and 13".

The slot elements extend outwardly from a central conducting region 17 and form at their outer edges 19, 19, etc., edges which may be assumed to lie on an imaginary line extending outwardly from some assumed point 20 so as to form an angle at with the axis of the conducting section 17 between the different slot elements. This angle, preferably, should be in the range between 5 and 50 for best operation, depending on the requirements of the application.

The physical shaping of the slotted array is such that the distance d or d etc., as the case may be, of the outer edges of the edges of that portion extending along the previously defined imaginary line is such that at the mid-point it may be considered as of a length (such as 13, 13, or the like) which will represent approximately a quarter wavelength (M4) at some selected frequency. This frequency will be determined by the length of the component or slot 13, 13, etc. The angle on may be varied within relatively wide limits, but for practical purposes the suggested range of between 5 and 50 has been found to be particularly suitable.

The various slot elements are spaced from each other along the connector or tfeeder 17 by a distance representing that between corresponding points on two adjacent elements, ill ustratively the elements 21, 23, 24 and 26. This spacing likewise is generally proportioned to the same scaling factor that determines the length of the various slot elements 21, 23, 24 and 26, for instance.

In a preferred embodiment the spacings bear a generally log-periodic relationship to each other. For practical reasons, in some instances, the relationship may not be strictly log-periodic, although the closer the approach to log-periodic spacing, the greater the antenna band width. To illustrate the relationship of the various components, it may be assumed that the scaling factor r=d /d as shown by this figure. Similarly, to determine the scaling factor along the feeder, the distances, illustratively, may be those measured between the centers of the slots such as distances s s and the like. Under these circumstances, the scaling factor to determine slot separation r=s /s If it is desired to have different scaling factors for the slot lengths as compared to the slot separations, those relationships may readily be provided with the understanding that the operation represents a compromise and the true log-periodic relationship does not obtain.

The antenna as described in the form of a slotted element in FIG. 1, it will be noted, is precisely symmetrical the points 29 and 30, so that reference to both parts is at this point unnecessary. The resonant frequency of each of the slots is determined by the slot length. A slot is termed resonant when its length is one-fourth the applied wavelength. The antenna radiates in a backfire fashion, which is from the high frequency end. It receives in the same fashion.

From what is shown from FIG. 1, it will be noted that each slot is provided with a central conducting section 31, 33, etc. In order that the slot sections may be correctly phased relative to each other, the structure shown embodies features which are not, so far as is known, common to any slot structure heretofore known. When a load circuit, such as a transmission line (not shown), is connected as at a point 35 and at a point 37, it will be observed that there is no direct connection between the transmission line conductors connected to these two points. This accounts for the fact that a whole series of elements of different resonant frequencies, as depicted, can be excited from a single feed line. Further than this, while an ordinary slot element has an impedance which is relatively high, the impedance for the folded symmetric structure here depicted is only only about one quarter this value. Under the circumstances, a better match to the usual transmission line impedance can readily be achieved.

Illustratively, the transmission line as the feeder or output 7 load conductor has a relatively low impedance of about 72 ohms. This can easily be matched by the structure disclosed. The currents on the central axis flow in the ground plane 11 generally parallel to the central axis of the array. This allows the ground plane to be out along the array axis, if desired, and printed circuit transmission line elements inserted therein.

It will be noted, however, from the arrangement of FIG. 1 that the parts of the center conductor, such as 31, 33, etc., within the slots have been symmetrically cut away and form a nonconducting region, such as 41, 43, etc. The center sections are all held by the illustrated insulating supports which span the slot elements (such as 21) and which, for convenience, are marked as INS and are designated by reference 42 in all regions. The slots provide what may be termed phasing slot elements. They serve to force the currents flowing along the central conductors to fio-w in the longer path in about the cut-away section 41 or 48, or the like, and, accordingly, provide an adjustable phasing mechanism. Basically, the length of the phasing slots, such as 41, 43, etc., should be about one-half the length of the outer slots. To this end, the slots may be considered as terminating along an imaginary line forming an angle a xe/2. Again, while the showing is for an optimum value, slight variances may be made depending on the application without materially changing the effectiveness of the operation. The combination may be used with equal facility for transmission or reception.

If reference now is made to FIG. 2, it will be observed that the combination is approximately the negative or opposite of that shown by FIG. 1. Under the circumstances, the entire assembly may be positioned and supported in air or it can be carried upon any desired in sulating surface (not shown), such as a sheet of nonconducting material of the variety commonly known by the trade name Lucite. The design of FIG. 1 can also be constructed of conducting rod and thus be self-supporting. For the illustrated conditions, the various folded dipole arrays, comprising the folded dipole sections 51, 53, 55, etc., connected respectively by feeder connectors 57, 59, etc., outward to two terminating points 61 and 63, may be formed by usual printed circuitry techniques on the non-conducting surface. As such, the outer edges of the folded dipoles lie along an imaginary path which can be assumed as extending at an angle ar relative to the central axis of the assembly. Generally speaking, with the substance of the different folded dipole arrays being generally of log-periodic relationship such that x /x =T, etc., the resonant frequency of each section of the folded dipo'le array will be determined by the distance from the mid-point of that part of the dipole array which extends along the imaginary line at an angle a, to the center. This, as already explained in connection with the slot arrangement of FIG. 1, is such that the distance x, is about onequarter wavelength at the frequency to which the particular folded dipole array section or element becomes resonant.

As was the case with the slot array and the central cut-away sections 41, 43, etc., the folded dipole array of FIG. 2 has centered therein conducting sections 65, 67, etc. These are arranged symmetrically relative to the axis, indicated by the dot-dash line, and are each approximately half the length of the associated folded section. The conducting elements provide phasing elements for control of the operation in a fashion complementing that described for slots of FIG. 1. The outer ends of the phasing elements, such as 65, 67, etc., in a preferred embodiment extend to an imaginary line which is at an angle a relative to the center line of the assembly.

The modification proposed by FIG. 3 provides a logperiodic folded monopole array. It will be observed that this arrangement is esentially a duplicate of one-half of the form of the invention pictured and described by FIG. 2. In this arrangement with an array of monopoles, the terminal point 75 is connected, illustratively, to the positive end of a coaxial cable array. A cable sheath is then connected to a ground terminal at which ground, as indicated at 79, the array of folded conductors terminates. The folded monopoles are provided in one form by a conductor 81 which is bent in suitable shape to provide the desired form of folded elements. As indicated by the drawing, the measured distance to the center of one of each of the folded elements to the ground connection is approximately one-quarter wavelength. In the form shown, the lower portions 83, 84 and 85 of the several folded arrangements are all suitably held above the ground connection and, therefore, insulated from the ground. The width of the separation between opposite sides of any of the folded structures, as illustratively indicated by the dimension x for one of the folded elements, can be generally anything desired. For practical purposes, the showing of the FIG. 3 drawings is general 1y reasonably schematic.

The relative spacing between the different elements is usually determined by the scaling factor for both the height and the spacing of the folded arrangements. Reference may be made to FIG. 2 and to the description thereof for this analysis. In this case, it may be borne in mind that the folded arrangement consists preferably of a continuous single conductor with two sides generally parallel and of lengths such that the bending position shall fall along the assumed angular path designated as ar In this connection, however, it may be noted that the angle ct is measured from a line just slightly above ground and out to the direction of the various folded elements. Under the circumstances, the angle 0c may be slightly less than was suggested for the forms of the invention depicted by FIGS. 1 and 2. The modification of FIG. 3 locates the phasing elements 87, '88, 89, etc., generally midway between the opposite side conductors of the folded dipole. So positioned, these elements function as phasing elements, such as 65, 67, etc., of FIG. 2. The phasing elements 87, 8'8 and 89 are each of a length about one-half of that of the folded elements forming the complete structure. The phasing elements are connected to the ground terminal 79 as indicated.

The structure of FIG. 3 may be positioned and supported upon an insulating support somewhat like that suggested for the FIG. 2 structure. In the alternative, the structure may be self-supporting and positioned over a conducting ground. To simplify an understanding of the invention and to ease the defining of the relationship of the components with respect to each other, a cell may be defined as comprising those components or parts of a folded array between two repeating symmetrical points. This is illustrated schematically by the components between the points identified on FIG. 3 by the letters A and B. It will be seen that, so arranged, a cell, illustratively, comprises that part of the feeder between the most lefthand folded arrangement and a similar point immediately above the feeder 84. As illustrated, the cell is shown to comprise a part of an upwardly extending conductor, but this is not in any respect essential. The measurement can be made from the point of the turn, as indicated at points 91 and 92, respectively, or it can be made from any other two correspondingly located points.

In the FIG. 3 arrangement, it is to be understood that the components are generally sufficiently rigid to be able to maintain their position without external support. However, in instances where less rigid components are utilized, the various elements may be suspended relative to the ground connection 79 by means of a support cable such as that schematically shown at 93 in FIG. 4. Nor-- mally the cable is of a non-conducting character. In many instances a so-called nylon rope type is satisfactory. The support cable is preferably hung by way of insulating connections, such as those schematically represented at 94, from supporting masts 95 and 96 which are securely supported in the ground plane 77. The continuous conductor 81' is supported by a plurality of insulating elements 98, 180, 98', 100', etc., which are carried at the lower ends of further cables, such as 101, 102, 103, 105, etc., which are hung from and supported by the main support cable 93 as connected thereto at points such as 106, 106, etc. Each if the insulating elements 98, 100, 98', 180 102, etc., is placed substantially adjacent to the conducting element 81 in order that the effective electrical length of the dipole sections may not be changed by the manner of support selected. The lower portion of the conductor 81 is insulated from the ground plane in any suitable manner, of which the insulators 112 to provide insulation and fix the location are suitable.

The phasing components, such as represented at 87 and 88, for instance, are also supported from the ground plane 77 and are held at their upper section by a cable, such as 105, 105", etc., each of which, in turn, is supported from the cable 93. Substantially adjacent to the upper portion of the phasing elements, an insulator 108, 108, 108", etc., is positioned so that a nonconducting termination of the phasing element is established at its supporting position. The feed connections for the structure are provided at terminals 75 and 77, as explained in connection with the arrangement of FIG. 3. The radiation pattern of the arrangement depicted is illustratively that shown by the dotted outlined half lobe 189.

Under some conditions, a rigid sheet conductor is undesirable to use. For such circumstances, a pair of less rigid cables may be provided, as shown by the pair of conductors 115 and 116 of FIG. 5, which continue from the terminal point 75 around to the terminal point 79, whereat a ground connection is made to the grounding point 77. The conductors are suspended from a support cable, as in the structure of FIG. 4, with the phasing elements 87, 88, and the like, similarly connected to that structure for FIG. 4. Preferably, the separate conductor-s 115 and 116 are electrically connected together at points of bending, such as shown at 119, 120, 121, etc., so that effectively there are pairs of conductors. The bent region of the folded combination is made to correspond to that of the remaining figures. The length of the conductors, as already explained in connection with FIGS. 1 and 2 in particular, is made so that the assembly follows essentially the logperiodic formation as previously described in connection With the preceding figures.

In some cases when the structures of either FIG. 4 or FIG. 5 are use, the support cables can be conducting, but this is usually not desirable because of the possible etfect of changing the electrical characteristics of the antenna structure per se, although proper location of the insulators usually precludles such likelihood.

A still further modification of the invention is depicted by FIG. 6, wherein a slot 127 corresponding substantially to the slot of FIG. 1 is cut into a conducting support plate 129. The slot is made approximately one-half wavelength long and has located in its center and held in insulating fashion relative to the metal support 129 a phase element 131. The phasing element may be supported within the slot by the conventionally illustrated insulating brackets 132 located at each corner. Centered on the slot is a conducting loop extending from a connecting point 133 on the conducting plate 129 around to a support point 134. Feeder connection is provided by connection, illustratively, made to a feed cable or the like having one terminal connected at the point 133 and the second terminal connected at the point 135 on the conducting phasing element 131. The arrangement of FIG. 6 is primarily that by which the combination of a loop and a slot may be simultaneously excited with properly related phase and magnitude pulses, thereby to make possible the cancellation of the far field of the slot in the chosen half space. In actual operation of the structure of the type shown in FIG. 6, at least three sheet metal loops, with a means circumference calculated substantially in accordance with the selected scaling factor, will be utilized to provide the desired type of logperiodic operation.

The modification of the structure shown by FIG. 8 is based generally upon the representation of FIG. 1, but to it adds an absorbing cavity structure. The cavity structure is particularly useful with flush-mounted antenna elements. The cavity is formed by a component, schematically represented as 151, which extends downwardly be- 10W the ground plane and houses the lower portion of the structure. Essentially, the absorbing cavity is formed from bottom and side walls, the bottom wall here being conventionally represented as a zigzag line 153. The walls are formed of some suitable absorbing structure that will convert into heat the energy of the residual electrical waves not perfectly cancelled by the conducting loops 11a and entering the cavity, so that there is no reflection from the cavity. An absorbing structure of this type is conveniently provided by a graphite impregnated foam rubber padding 151 which extends in such a way as to house completely the folded slot arrangement. The wall structure extends at one end to substantially the ground plane and then extends downwardly therefrom in such fashion as to enclose all of the loops of the complete slot array.

The conducting loops 11a are in series with the feed line structure, and progressively increase in diameter from the feed point back in a logarithmically periodic fashion as previously described. The scale factor governing the diameter of the loops may or may not be equal to the scale factor governing the slot antenna structure. Furthermore, the antenna of FIGURE 8, can function without the loops 11a, although a decrease in efficiency will result.

The structure schematically represented is one where a loop antenna is shown to be excited by the slot antenna,

such as that of FIG. 1. In this case, the far-field patterns of the slot and loop antennas are the same. Consequently, if the loop and slot can be excited out of phase, the resultant fields will cancel on one side of the ground plane. It is recognized, in this circumstance, that in receiving with this type of structure some minor losses may occur, but these are completely insignificant. In the particular form of structure here suggested, the loop and the slot are arranged in series thereby to force all of the current through the loop.

A. further modification of the monopole arrangement shown by FIG. 4 is provided with the structure of FIG. 7. In this form the connected U-shaped members forming the teeth sections of the monopoles which are connected by the continuous conductor 81 are replaced by a triangularly arranged assembly provided by the designated conductors 161 which terminate at one end at the ground connection 77 and which at the apex at each of the triangles connect to the support cable 93. An insulator (such as 98, 98', etc.) is arranged at the apex so that the conductors are held somewhat in the general fashion of those represented in FIG. 4. The lower portion of each triangular section connects to the next smaller, etc., with the feed connection being as provided by the terminal connections and 77. The advantage of the FIG. 7 connection as compared to that represented by FIG. 4 resides substantially in its simplicity of installation. In this construction of the monopole antenna, the triangular teeth sections each form substantially the equal length sides of an isosceles triangle. The third side of such triangle is actually missing but, if present, would form. its base. This third dimension is actually usually less than the other two sides. There is a. feeder between adjacent teeth sections. The feeder lies substantially in a plane of what would constitute the third ore short side of the triangle. The feeder is parallel to ground or some plane of fixed or uniform potential. The separate triangles are supported substantially at the apex formed at the junction of the two substantially equal length sides.

Various other modifications are, of course, within the spirit and scope of the invention here set forth and disclosed.

Having now described the invention, what is claimed is:

1. A wide-band antenna structure comprising a plurality of folded elements positioned in a sequence parallelly and coplanarly, each of the elements being of different electrical length, an electrical transmission line section connecting adjacent elements, the transmission line sections collectively providing a feeder between the plurality of elements, means to couple each of the elements to the transmission line for feeding energy thereto, the outer ends of the plurality of elements terminating along a path extending outwardly at an angle in the range between about 5 and 50 from the transmission line, a phasing means centrally located within each folded element and extending outwardly substantially parallel to the element with successive phasing means terminating at a path extending at an angle which is in the range between the angle forming the path along which the folded elements terminate, and means to connect a load circuit to the folded elements.

2. A wide-band antenna structure comprising a plurality of series-fed folded elements positioned in a sequence parallelly and coplanarly, each of the elements being of different electrical length and orderly changing in accordance with a selected scale factor, electrical transmission line sections connecting adjacent elements, the transmission line sections collectively providing a feeder between the elements of the plurality, the inner ends of each of the elements being connected to the transmission line and the outer ends of each of the elements terminating along a path extending at an angle from the central longitudinal axis of the transmission line, said angle being selected in the range between about 5 and 50, a phasing means centrally located within each folded element and extending outwardly from the central axis, the phasing elements successively terminating on a path extending at an angle from the axis which is greater than 0 and less than the angle forming the terminating path of the folded elements, and means to connect a load circuit to the folded elements. I

3. The antenna structure claimed in claim 2 in which the phasing element is electromagnetically coupled to the folded element in which it is located.

4. A wide-band antenna structure comprising a plurality of series-fed folded dipole elements positioned in a sequence substantially parallelly and coplanarly, the elements varying in regularly changing electrical lengths from one end of the structure to the other with successive dipoles being shorter in accordance with a scale factor smaller than unity, an electrical transmission line section connecting adjacent elements, the transmission line sections collectively providing a feeder between the plurality of elements, a connection from the inner ends of each of the elements to the transmission line, the outer ends of each of the elements terminating along a path extending at an angle from the central longitudinal axis of the transmission line, the angle being selected in the range between about and 50, a phasing means centrally located within each folded element and extending outwardly from the transmission line, the phasing elements successively terminating at a path extending at an angle from the axis, which angle is greater than 0 and less than the angle forming a path along which the folded elements terminate, and means to connect a load circuit to the transmission line in the region of the smallest of the elements.

5. The antenna claimed in claim 4 wherein the dipoles are spaced along the transmission line section With spacing variances from the largest to the smallest being substantially according to a scale factor having a value less than unity.

*6. The antenna structure claimed in claim 4 wherein the dipole elements change in length from the longest t0 the shortest substantially logarithmically in accordance with a selected scale factor, and wherein the spacings of the folded dipole elements from the longest element to the shortest also change from the longest to the shortest in accordance with the same scale factor.

7. An antenna structure for Wide-band operation comprising a folded monopole arrany having at least three folded monopole elements, each folded monopole including substantially parallel opposite side members and an outer end closure element, a transmission line section between adjacent folded monopoles, a connection from each monopole side member to the transmission line so that the transmission line and the folded monopoles are serially connected, means to connect the outer end terminal of the largest folded monopole to a plane of fixed potential, and a phasing element contained within each folded monopole, each phasing element being connected at one end to the plane of fixed potential, the length of adjacent monopoles varying in uniform direction according to a selected scale factor from one end of the sequence to the other, the spacing between adjacent folded mono- 10 poles also varying in accordance with a selected scale factor from one end to the other;

8. The antenna structure claimed in claim 7 wherein the monopole elements change in length from the longest to the shortest substantially logarithmically in accordance with a selected scale factor, and wherein the spacings of the folded monopole elements from. the longest element to the shortest also change from the longest to the shortest in accordance with the same scale factor.

9. The combination claimed in claim 1 comprising, in addition, an absorbing cavity positioned adjacent to the folded elements thereby to direct antenna response to a region remote from the cavity.

10. An antenna structure for Wide-band operation comprising a folded monopole array having at least three folded monopole elements, each folded monopole including opposite side members defining a triangularly shaped tooth and an outer end closure element, a transmission line section between adjacent folded monopoles, a connection from each monopole side member to the transmission line so that the transmission line and the folded monopoles are serially connected, means to connect the outer end terminal of the largest folded monopole to a plane of fixed potential, and a phasing element contained Within each folded monopole, each phasing element being connected at one end to the plane of fixed potential, the length of adjacent monopoles varying in uniform direction according to a selected scale factor from one end of the sequence to the other, the spacing between adjacent folded monopoles also varying in accordance With a selected scale factor from one end to the other.

11. The antenna structure claimed in claim 10* wherein each triangular tooth is substantially isosceles and including a support means secured to the triangle apex opposite the short base.

References Cited UNITED STATES PATENTS 2,888,678 5/1959 Weiss et al. 343-Q X 3,165,748 1/ 1965 Woloszcuk 343--792.5 3,212,094 10/ 1965 Berry 343792.5 3,276,027 9/ 1966 Bell et al 343-7925 3,286,268 11/1966 Barbano 343792.5

ELI LIEBERMAN, Primary Examiner. 

